1. Field of the Invention
The present invention relates to a fuel-cell system and a method of controlling a fuel cell, and more particularly, to a fuel-cell system and a method of controlling a fuel cell mounted on automotive vehicles.
2. Related Art
In recent years, fuel-cell systems gain the spotlight as a new power source of automotive vehicles. For example, a fuel-cell system includes a fuel cell for generating power by chemical reaction of reactive gases, a reactive-gas supply unit for supplying the reactive gases to the fuel cell through a reactive-gas passage, and a control unit for controlling the reactive-gas supply unit.
The fuel cell has a stack structure in which tens to hundreds of cells are stacked, for example. Each cell has a membrane electrode assembly (MEA) interposed by a pair of separators. The MEA includes two electrodes, i.e., an anode electrode and a cathode electrode, and a solid high-polymer electrolyte membrane interposed by the electrodes.
When supplying hydrogen gas as a reactive gas to the anode electrode and air containing oxygen as a reactive gas to the cathode electrode, the fuel cell generates power by electrochemical reaction. Since only harmless water is fundamentally produced during this power generation, the fuel cells gain the spotlight from the viewpoint of influence on the environment and efficiency.
In the fuel-cell system, the reactive gases such as hydrogen gas, air, etc. are supplied to one end of a casing in which the cells of stacked structure are accommodated, and are discharged from the other end. Therefore, when supplying an amount of reactive gases required for power generation, a sufficient amount of reactive gases is supplied to the cells in the vicinity of an inlet of the reactive gases, but not to the cells in the vicinity of an outlet of the reactive gases, leading to possible occurrence of variation in the power-generation efficiency of the cells.
In order to efficiently generate power in all the cells, a greater amount of reactive gases than a theoretical amount required for power generation is supplied to the fuel cell.
Specifically, assuming that a minimally required amount of reactive gases calculated in accordance with a required amount of power generation is a required amount of reactive gases, a rate of an amount of reactive gases supplied to the fuel cell with respect to the required amount of reactive gases is set as a stoichiometric flow ratio. The reactive gases are supplied so that the stoichiometric flow ratio has a constant value.
However, even if the stoichiometric flow ratio is set to a constant value, variation may occur in the power-generation efficiency of the cells due to influence of operation loads and secular changes.
In order to solve this problem, a system is proposed wherein when variation occurs in the generated voltage of the cells constituting the fuel cell, it is determined whether the cause lies in deviation of the distribution of reactive gases in the fuel cell or the degradation of the cells, and a target stoichiometric flow ratio of reactive gases is set in accordance with the result of this determination (refer to Japanese Unexamined Patent Application Publication No. 2004-207029 (Patent Document 1)).
With the fuel-cell system proposed by Patent Document 1, when the cause of variation in generated voltage of the cells lies in deviation of the distribution of reactive gases, a slightly greater stoichiometric flow ratio than an optimal stoichiometric flow ratio is set as a target value in consideration of safety factors. With this, a sufficient amount of reactive gases can be supplied to the fuel cell, achieving stabilization of the generated voltage and an enhancement in fuel consumption.
With the system of Patent Document 1, however, since the stoichiometric flow ratio is slightly greater than the optimal stoichiometric flow ratio that can stabilize the generated voltage, power consumption of a compressor for supplying air to the cells increases, leading to a possible reduction in fuel efficiency of the fuel cell.
In order to solve this problem, a fuel-cell system is desired in which the stoichiometric flow ratio can be reduced as much as possible to allow a further enhancement in fuel consumption. Therefore, a fuel-cell system is proposed in which the stoichiometric flow ratio of reactive gases is changed dynamically, and the voltage of the fuel cells is detected to determine whether or not a current stoichiometric flow ratio is appropriate to stably maintain the voltage of the fuel cell (refer to Japanese Unexamined Patent Application Publication No. 2005-093218 (Patent Document 2)).
According to the fuel-cell system proposed by Patent Document 2, whether or not the stoichiometric flow ratio is appropriate is determined in accordance with the voltage of the fuel cell when the stoichiometric flow ratio of reactive gases is changed. Therefore, by reducing the stoichiometric flow ratio while observing the stability of power generation of the fuel cell, it is possible to maintain the stability of power generation and reduce the stoichiometric flow ratio as much as possible, leading to an increase in the fuel efficiency of the fuel cell.
However, in order to reduce the stoichiometric flow ratio as much as possible, the stoichiometric flow ratio should frequently be lowered to such an extent that power generation becomes unstable, resulting in an unstable power-generation capacity of the fuel cell.